Idiotic square like one before (7)
I believe the answer is:
asinine
'idiotic' is the definition.
(thesaurus)
'square like one before' is the wordplay.
'square' becomes 'nine' (I've seen this before**).
'like' becomes 'as'.
'one' becomes 'i' (Roman numeral).
'before' is a charade indicator (letters next to each other).
'as'+'i'='asi'
'nine' after 'asi' is 'ASININE'.
(Other definitions for asinine that I've seen before include "Daft" , "Stupid (as a donkey?)" , "Foolish, silly" , "'Mindless, vacuous (7)'" , "Silly like a donkey" .)